Compound semiconductor light emitting device and method of fabricating the same

ABSTRACT

Compound semiconductor light emitting devices capable of suppressing the surface state density on the facets of semiconductor light emitting devices such as semiconductor lasers for a long time and stable operating even when the passivation layer diffuses can be easily obtained. Compound semiconductor light emitting devices with an emission wavelength of λ (nm) wherein a first conduction type of clad layer, an active layer and a second conduction type of clad layer are grown on a substrate and two facets are opposite to each other so as to form a cavity, characterized in that said active layer is transparent to the emission wavelength in the vicinities of the facets and that the surfaces of the first conduction type of clad layer, active layer and second conduction type of clad layer forming said facets are each coated with a passivation layer.

This application is a division of application Ser. No. 09/095,884, filedJun. 11, 1998, now U.S. Pat. No. 6,323,052.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor light emitting device and amethod of fabricating the same, in particular, a process for producingsemiconductor lasers. The device and the process according to thepresent invention are usable appropriately in semiconductor lasers whichshould have high light output power and high reliability, for example,excitation light sources for optical fiber amplifier and light sourcesfor optical data storage system. Moreover, the device and the process ofthe present invention are applicable to LED of super-luminescent diodes,etc. wherein the facet of the light emitting device serves the lightemission vertical cavity surface emitting lasers, etc.

2. Description of the Related Art

In recent years, optical data processing technology and opticalcommunication technology have achieved brilliant extraordinary resultsexemplified by high-density recording with the use of optical magneticdiscs and two way communication with optical fiber networks.

In the communication technology, for example, studies have beenenergetically made in various fields to develop large-capacity opticalfiber transmitters usable in the coming multimedia age as well asEr³⁺-doped optical fiber amplifiers (EDFA) as signal amplifiers flexiblyapplicable to these transmission systems. Under these circumstances, ithas been required to develop semiconductor lasers with high output powerand high reliability which are essentially required as a component ofEDFA.

The emission wavelengths usable in EDFA theoretically include thefollowing three wavelengths, i.e., 800 nm, 980 nm and 1480 nm. By takingthe characteristics of amplifiers into consideration, it is known thatexcitation at 980 nm, among all, is most desirable from the viewpointsof amplifier efficiency, noise figure, etc. It is needed that laserswith the emission wavelength of 980 nm have two contrary characteristicsof high output and high reliability. Moreover, there are demands forlasers with wavelength in the vicinities thereof (for example, 890-1150nm) as light source of secondary harmonic generation (SHG) and a sourceof thermal laser printers. In addition thereto, it has been urgentlyrequired to develop highly reliable lasers with high output in variousfields.

In the field of data processing, attempts have been made to increase theoutput and shorten the wavelength of semiconductor lasers to achievehigh-density recording and rapid writing and reading. That is to say, ithas been strongly required to increase the output of laser diodes(hereinafter referred to simply as LDs) with the conventional emissionwavelength of 780 nm and studies have been energetically made to developLDs of 630 to 680 nm.

To achieve both of high output and high reliability which areessentially required in these lasers, there have been proposed a numberof methods, for example, one comprising making the band gap in theactive layer region around the facets so as to suppress the lightabsorption in the vicinities of the facets. Lasers with thesestructures, which are generally called window-structure lasers or nonabsorbing mirror (NAM)-structure lasers, are highly effective inestablishing high output laser diodes.

On the other hand, JP-A-3-101183 (the term “JP-A” as used herein meansan “unexamined published Japanese patent applications”) proposes anothermethod for solving the above problem. According to this patent, it iseffective to form a contamination-free facet and then form a passivationlayer or a part of the same with the use of a material which undergoesneither any reaction with the semiconductor facet nor diffusion per seand contains no oxygen.

As known reference similar to the above patent JP-A-3-101183, citationmay be made of L. W. Tu et al., In-vacuum cleaving and coating ofsemiconductor laser facets using Si and a dielectric, J. Appl. Phys. 80(11) Dec. 1, 1996. According to this paper, when cleavage is performedin vacuum in the step of coating laser facets with an Si/AlO_(x)structure, the carrier recombination in the cleaved facet is retardedand thus the initial catastrophic optical damage (COD) level isincreased.

Further, there has been known a technique for inserting an Si layerhaving an optical thickness corresponding to ¼ of the emissionwavelength between a coating film and each semiconductor layer so as todisplace the facets from the anti-node of the standing wave in thedirection of the cavity, thus lowering the electric field strength atthe beam emission facet.

For example, there have been already developed semiconductor lasers of980 nm or therearound and withstanding continuous use for about 2 yearsat light output of 50 to 100 mW and a process for producing the same.When operated at higher light outputs, however, these lasers are rapidlydegraded, thus showing poor reliability. The same applies to LDs of 780nm or 630-680 nm. Thus, it is the problem which now confronts allsemiconductor lasers, in particular, those with the use of GaAssubstrates to ensure a high reliability at higher output.

One of the reasons therefor resides in the degradation of the diodefacet exposed to extremely high light output density. As well knownregarding GaAs/AlGaAs semiconductor lasers, there are a number ofsurface states (gap state) in the vicinities of facet. Since thesestates absorb the output light, the temperature in the vicinities of thefacets is generally higher than the temperature at bulk of the laser.This increase in temperature further narrows the band gap in thevicinities of the facets and then the output light can be more easilyabsorbed, thus causing a positive feedback. This phenomenon is known asso-called COD observed when a large current is injected instantly. Onthe other hand, there arises a problem, in common to a number ofsemiconductor laser elements, of the sudden failure associating adecrease in the COD level after long time driving. Although attemptshave been vigorously made to solve these problems as described above,the technical level at the present stage is insufficient.

An LD with the window-structure can be produced by, for example,epitaxially growing a semiconductor material transparent to the emissionwavelength on the laser facets. In this method, the epitaxial growth isperformed on the facets while making the laser in the so-called barstate, which makes the subsequent electrode step highly troublesome.

Furthermore, there are various methods which comprise intentionallythermal-diffusing or ion-implanting Zn, Si, etc. as impurities into anactive layer in the vicinities of laser facets so as to disorder theabove-mentioned active layer, as proposed by JP-A-2-45992, JP-A-3-31083and JP-A-6-302906.

During the production of an LD, impurities generally diffuse in theepitaxial growth direction of the laser element toward the substrate.Accordingly, there arise problems in controlling the diffusion depth andcontrolling the horizontal diffusion to the cavity direction, whichmakes stable production difficult.

When ion-implantation is carried out, ions with high energy areintroduced from the facets. As a result, damages frequently remain onthe LD facets even after annealing. Moreover, there arise anotherproblem that an increase in the reactive current accompanying thedecrease in the resistance in the region into which impurities have beenintroduced would increase the threshold current and driving current.

On the other hand, the process disclosed in JP-A-3-101183 as citedabove, which comprises forming a contamination-free facet and thenforming a passivation layer or a part of the same with the use of amaterial which undergoes neither any reaction with the semiconductorfacet nor diffusion per se and contains no oxygen, suffers fromtechnical problems as will be described below.

It is generally impossible to prevent the formation of non-radiativerecombination centers such as Ga—O and As—O on the facet at cleavage byperforming the operation in the atmosphere in, for example, a cleanroom. From this point of view, it is essentially required to form apassivation layer in situ at the point of cleaving for the “method offorming a contamination-free facet” as disclosed in Claim 1 in thegazette of this patent. To embody this method in practice, the cleavageshould be carried out in vacuum as stated in Claim 10 in the gazette.For an effect cleavage in vacuum, an extremely complicated procedure andtroublesome labor are required in general, compared with the case wherecleavage is effected in the atmosphere. Many non-reactive recombinatioricenters are formed on facets formed by dry-etching as stated in Claims11 to 14 in the gazette, compared with facets formed by cleavage. Thus,this dry etching procedure is unsuitable for the production of LDs whichshould have high reliability.

The optimum examples of the passivation layer are Si (single crystal orpolycrystal Si) and amorphous Si. However, there is no substance neverundergoing diffusion in general. In semiconductor lasers which are to beoperated at high output and high temperature for a long time, inparticular, it is feared that the passivation materials disclosed in theabove patent might diffuse.

Although it is described in L. W. Tu et al., In-vacuum cleavage andcoating of semiconductor laser facets using Si and a dielectric, J.Appl. Phys. 80 (11) Dec. 1, 1996 as cited above that when an Si/AlO_(x)structure is cleaved in vacuum in the step of coating onto a laserfacet, the carrier recombination on the cleaved facet is retarded andthus the initial COD level is increased. However, this reference refersto neither reliability over a long time nor the relationship betweencoating and the LD structure.

Further, there has been known a technique for inserting an Si layerhaving an optical thickness corresponding to ¼ of the emissionwavelength of between a coating film and each semiconductor layer so asto displace the facets from the anti-node of the standing wave in thedirection of the cavity, thus lowering the electric field strength atthe beam emission facet. However, this technique suffers from a fearthat Si per se would serve as a light absorption in the emissionwavelength region embodied by usual semiconductor lasers, in particularwithin the range of from 400 to 1600 nm needed for high-output LDs andthus there is a possibility that the degradation of devices might beaccelerated by an increase in temperature on facets.

SUMMARY OF THE INVENTION

An object of the present invention, which has been completed to solvethe above problems, is to provide semiconductor lasers capable ofsuppressing the surface state density on the facets of semiconductorlight emitting devices such as semiconductor lasers for a long time andstable operating even when the passivation layer diffuses, and a processfor conveniently producing the same. In other words, the presentinvention provides high-performance semiconductor lasers establishingboth of high output and high reliability by preventing degradation onfacets.

The present inventors have found out that semiconductor light emittingdevices having compound semiconductor layers containing at least a firstconduction type of clad layer, an active layer transparent to theemission wavelength in the vicinities of the facets and a secondconduction type of clad layer formed on a substrate and having cavityfacets coated with passivation layers are much superior in high outputand high reliability to the conventional ones, thus completing thepresent invention.

Accordingly, the gist of the present invention resides in a compoundsemiconductor liglit emitting device having a compound semiconductorlayer containing at least a first conduction type of clad layer, anactive layer and a second conduction type of clad layer formed on asubstrate and have a cavity facets, characterized in that the activelayer is transparent to the emission wavelength in the vicinities of thefacets, preferably free from oxide, and that the facets are coated witha passivation layer.

The present inventors have also found out that semiconductor lightemitting devices having a p-type active layer preferably containing Inand having a cavity facet coated with a passivation layer containing Si,more preferably having disordered cavity facet free from oxide are muchsuperior in high output and high reliability.

Accordingly, the gist of the present invention resides in compoundsemiconductor light emitting devices wherein a first conduction type ofclad layer, an active layer and a second conduction type of clad layerare grown on a substrate and two facets opposite to each other form acavity, characterized in that said conduction type of active layer is Pand that the surfaces of the first conduction type of clad layer, activelayer and second conduction type of clad layer forming said facets areeach coated with a passivation layer containing Si.

The present inventors have further conducted extensive studies to solvethe above-mentioned problem. As a result, they have found out that whensaid facets are treated by irradiating with plasma having energy fallingwithin an optimized range from the facet side, the facets can be easilymade transparent without any problems (for example, control of thediffusion depth and the horizontal diffusion to the cavity as observedin the case where impurities are diffused; difficulties in the electrodeformation as observed in the case where epitaxial growth is performed onthe facets; passage of reactive current accompanying the decrease in theresistance in the vicinities of the facets), thus giving semiconductorlasers achieving both of high output and high reliability. The presentinvention has been completed on the basis of this finding.

Accordingly, the gist of the present invention resides in a process forproducing a semiconductor laser having a first conduction type of cladlayer, an active layer and a second conduction type of clad layer formedon a semiconductor substrate, characterized in that at least one facetforming a cavity is irradiated with plasma having energy of from 25 eVto 300 eV.

The present inventors have further conducted extensive studies to solvethe above-mentioned problem. As a result, they have found out thatsemiconductor light emitting devices produced by a process for producinga semiconductor light emitting device having a compound semiconductorlayer containing at least a first conduction type of clad layer, anactive layer and a second conduction type of clad layer formed on asubstrate and having a cavity, which comprises forming the compoundsemiconductor layer on the substrate by crystal growth; next forming thecavity facets; then desorping a part of the elements constituting atleast the active layer in the vicinities of the facets exposed on atleast one of the facets via irradiation with ion, electron, heat and/orlight, etc. to thereby form a region transparent to the emissionwavelength in the vicinities of the semiconductor light emitting device;and forming a passivation layer in vacuum; are much superior both inhigh output and high reliability to the conventional ones, though theproduction process is highly convenient, thus completing the presentinvention.

Accordingly, the gist of the present invention resides in a process forproducing a semiconductor light emitting device having a firstconduction type of clad layer, an active layer and a second conductiontype of clad layer formed on a substrate and having a cavity,characterized by comprising forming the compound semiconductor layer onthe substrate by crystal growth; next forming the cavity facets; thendesorption of a part of the elements constituting at least the activelayer in the vicinities of the facets exposed on at least one of thefacets; and forming a passivation layer in vacuum.

The present inventors have further conducted extensive studies to solvethe above-mentioned problem. As a result, they have found out that, in asemiconductor laser having a first conduction type of clad layer, anactive layer containing quantum well and a second conduction type ofclad layer on a semiconductor substrate, elements with high vaporpressure in the vicinities of the facets can be selectively desorbed byirradiating ion, electron, light and/or heat in vacuum to at least onefacet forming the cavity. Thus a region having a band gap exceeding theeffective band gap of the materials constituting the active layer isformed in the vicinities of the facet. That is to say, the region ismade transparent to the emission wavelength of the semiconductor laser.Thus, high-performance semiconductor lasers with the window-structurecapable of achieving both high output and high reliability can be easilyobtained without suffering from the above problems encountering in theprior art. The present invention has been thus completed.

Accordingly, the gist of the present invention resides in asemiconductor laser having a first conduction type of clad layer, anactive layer containing quantum well and a second conduction type ofclad layer formed on a semiconductor substrate, characterized in thatthe active layer is made transparent to the emission wavelength bydesorping a part of the constituting elements in the vicinities of atleast one facet forming the cavity.

Another gist of the present invention resides in a process for producinga semiconductor laser having a first conduction type of clad layer, anactive layer containing quantum well and a second conduction type ofclad layer formed on a semiconductor substrate, characterized bycomprising forming the first conduction type of clad layer, active layercontaining quantum well and second conduction type of clad layer on thesemiconductor substrate; and selectively desorping elements with highvapor pressure by irradiating in vacuum at least one facet forming thecavity with ion, electron, light and/or heat beam to thereby form aregion which has been made transparent to the emission wavelength in thevicinities of the facet of the active layer.

The present inventors have further conducted extensive studies to solvethe above-mentioned problem. As a result, they have found out that acompound semiconductor light emitting device having at least a firstconduction type of clad layer, an active layer and a second conductiontype of clad layer grown on a substrate, two facets which are oppositeto each other forming a cavity and having an emission wavelength of λ(nm), characterized in that the surfaces of the first conduction type ofclad layer, active layer and second conduction type of clad layerforming the transparent facets are coated with passivation layers, andthe surfaces of the passivation layers are coated with a coating layercomprising a dielectric material optionally combined with asemiconductor, is much superior in high output and high reliability tothe conventional ones, thus completing the present invention.

Accordingly, the gist of the present invention resides in a compoundsemiconductor light emitting device having at least a first conductiontype of clad layer, an active layer and a second conduction type of cladlayer grown on a substrate, two facets which are opposite to each otherforming a cavity and having an emission wavelength of λ (nm),characterized in that the surfaces of the first conduction type of cladlayer, active layer and second conduction type of clad layer forming thefacets are coated with passivation layers made of Si, and the surfacesof the passivation layers are coated with a coating layer comprising adielectric material optionally combined with a semiconductor.

A first aspect of the device is a compound semiconductor light emittingdevice of present invention, which comprises

a first conduction type of clad layer;

an active layer; and

a second conduction type of clad layer grown on a substrate and

two of said first conduction type of clad layer; said active layer, andsaid second conduction type of clad layer, being opposite to each otherso as to form a cavity,

wherein said active layer is transparent to the emission wavelength inthe vicinities of the facets and the surfaces of the first conductiontype of clad layer, active layer and second conduction type of cladlayer forming said facets are each coated with a passivation layer.

A second aspect of the device is a compound semiconductor light emittingdevice according to the first aspect, wherein at least one of theconstituting elements of the surfaces of the first conduction type ofclad layer, active layer and second conduction type of clad layerforming said facets exists in the form free from an oxide.

A third aspect of the device is a compound semiconductor light emittingdevice according to the first aspect, wherein the vicinities of saidfacets have been disordered.

A fourth aspect of the device is a compound semiconductor light emittingdevice according to the first aspect, wherein a coating layer comprisinga dielectric material optionally combined with a semiconductor materialis formed on the surface of said passivation layer.

A fifth aspect of the device is a compound semiconductor light emittingdevice according to the first aspect, wherein said passivation layercontains Si.

A sixth aspect of the device is a compound semiconductor light emittingdevice according to the first aspect, wherein one of said facets iscoated with an anti-reflective coating layer containing an AlO_(x) layerwhile the other is coated with a high-reflective coating layercontaining AlO_(x) layer and Si layer.

A seventh aspect of the device is a compound semiconductor lightemitting device according to the first aspect, wherein said active layercomprises a compound semiconductor layer containing In.

An eighth aspect of the device is a compound semiconductor lightemitting device of the present invention, which comprises:

a first conduction type of clad layer; an active layer; and a secondconduction type of clad layer, grown on a substrate and

two facets of said first conduction type of clad layer; said activelayer; and said second conduction type of clad layer, being opposite toeach other so as to form a cavity,

wherein a conduction type of said active layer is p type and thesurfaces of the first conduction type of clad layer, active layer andsecond conduction type of clad layer forming said facets are coated witha passivation layer containing Si.

A ninth aspect of the device is a compound semiconductor light emittingdevice according to the eighth aspect, wherein at least one of theconstituting elements of the surfaces of the first conduction type ofclad layer, active layer and second conduction type of clad layerforming said facets exists in the form free from an oxide.

A tenth aspect of the device is a compound semiconductor light emittingdevice according to the eighth aspect, wherein the vicinities of thefacets of the cavity have been disordered.

A eleventh aspect of the device is a compound semiconductor lightemitting device according to the eighth aspect, wherein said activelayer comprises a compound semiconductor layer containing In.

A twelfth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device of the present invention, whichcomprises the steps of:

growing a first conduction type of clad layer, an active layer and asecond conduction type of clad layer on a substrate;

forming facets of a cavity; and

irradiating said facets of the cavity with plasma having energy of from25 eV to 300 eV in vacuum.

A thirteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the twelfth aspect,wherein step of forming facets of a cavity comprises a cleavage of thefirst conduction type of clad layer, the active layer and the secondconduction type of clad layer so that two facets are opposite to eachother to form the cavity.

A fourteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the twelfth aspect,wherein plasma of an element of the group 18 is used in said irradiatingstep.

A fifteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the twelfth aspect,which further comprises a step of:

forming a passivation layer on each facet after said plasma irradiationstep.

A sixteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the twelfth aspect,which further comprises a step of: forming an anti-reflective coatingand/or a high-reflective coating on said facets while evacuatingcontinuously after the irradiating step.

A seventeenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device of the present invention, whichcomprises the steps of:

growing a first conduction type of clad layer, an active layer and asecond conduction type of clad layer on a substrate;

forming facets of a cavity by cleavage of the first conduction type ofclad layer, the active layer and the second conduction type of cladlayer so that two facets are opposite to each other so as to form acavity;

removing, from said facets, a part of elements constituting the facet;and

forming a passivation layer on each facet after said removing step.

An eighteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the seventeenth aspect,wherein said removing step comprises a step of irradiating said facetswith at least one selected from the group consisting of ion, electron,light and heat in vacuum.

A nineteenth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the seventeenth aspect,wherein plasma having energy of from 25 eV to 300 eV is used in saidirradiating step.

A twentieth aspect of the method is a method of fabricating a compoundsemiconductor light emitting device according to the seventeenth aspect,wherein plasma of an element of the group 18 is used in said irradiatingstep.

A twenty-first aspect of the method is a method of fabricating acompound semiconductor light emitting device according to theseventeenth aspect, which further comprises a step of forming, on saidpassivation layer, a coating layer containing at least one combinationsof dielectrics and semiconductors.

A twenty-second aspect of the method is a method of fabricating acompound semiconductor light emitting device according to theseventeenth aspect, wherein said passivation layer contains Si.

A twenty-third aspect of the method is a method of fabricating acompound semiconductor light emitting device according to thetwenty-first aspect, wherein, at the formation of said coating layer,the surface is irradiated with plasma simultaneously with the formationof the coating layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the semiconductor light emitting deviceof the first example of the present invention.

FIG. 2 is a sectional illustration viewing from the cavity facet of thesemiconductor laser of the example of the present invention.

FIG. 3 shows the results of the life test (200 mW, output, 50° C., inAPC mode) on the semiconductor laser of Example 1 of the presentinvention.

FIG. 4 shows the results of the life test (200 mW, output, 50° C., inAPC mode) on the semiconductor laser of Example 2 of the presentinvention.

FIG. 5 shows the results of the life test (200 mW, output, 50° C., inAPC mode) on the semiconductor laser of Comparative Example 1 of thepresent invention.

FIG. 6 shows the results of the life test (200 mW, output, 50° C., inAPC mode) on the semiconductor laser of Comparative Example 2 of thepresent invention.

FIG. 7 shows the results of the life test (200 mW, output, 50° C., inAPC mode) on the semiconductor laser of Comparative Example 3 of thepresent invention.

FIG. 8 shows the results of the life test (100 mW, output, 50° C., inAPC mode) on the semiconductor laser of Example 3 of the presentinvention.

FIG. 9 shows the results of the life test (250 mW, output, 50° C., inAPC mode) on the semiconductor laser of example 4 of the presentinvention.

FIG. 10 shows the results of the life test (100 mW, output, 50° C., inAPC mode) on the semiconductor laser of comparative Example 4 of thepresent invention.

FIG. 11 is a graph showing the current VS optical output characteristicsof the semiconductor laser of Example 5 of the present invention.

FIG. 12 is a graph showing the results of the life test (200 mW output,70° C., in APC mode) on the semiconductor laser of Example 5 of thepresent invention.

FIG. 13 is a graph showing the initial current VS optical outputcharacteristics of the semiconductor laser of Comparative Example 5.

FIG. 14 is a graph showing the results of the life test (200 mW output,70° C., in APC mode) on the semiconductor laser of Comparative Example5.

FIG. 15 is a graph showing the initial current VS optical outputcharacteristics of the semiconductor laser of Comparative Example 6.

FIG. 16 is a graph showing the results of the life test (200 mW output,70° C., in APC mode) on the semiconductor laser of Comparative Example6.

FIG. 17 is a graph showing the initial current VS optical outputcharacteristics of the semiconductor laser of Comparative Example 7.

FIG. 18 is a graph showing the results of the life test (150 mW output,50° C., in APC mode) on the semiconductor laser of Example 6 of thepresent invention.

FIG. 19 is a graph showing the results of the life test (150 mW output,70° C., in APC mode) on the semiconductor laser of Example 7 of thepresent invention.

FIG. 20 shows the results of the life test (150 mW output, 70° C., inAPC mode) on the semiconductor laser of Comparative example 8 of thepresent invention.

FIG. 21 shows the results of the life test (150 mW output, 70° C., inAPC mode) on the semiconductor laser of Comparative example 9 of thepresent invention.

FIG. 22 shows the results of the life test (150 mW output, 70° C., inAPC mode) on the semiconductor laser of Comparative Example 10.

FIG. 23 shows the results of the life test (150 mW output, 70° C., inAPC mode) on the semiconductor laser of Comparative Example 11.

FIG. 24 is a perspective view of the semiconductor laser with adisordered and high resistant region of the present invention.

FIG. 25 is a perspective view of the active region, which is transparentto emission wavelength, of semiconductor laser of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Next, the present invention will be described in greater detail.

The structure of the semiconductor light emitting device of the presentinvention is not particularly restricted, so long as it is a compoundsemiconductor light emitting device wherein, as shown in FIG. 1, atleast a first conduction type of clad layer, an active layer and asecond conduction type of clad layer are grown on a substrate and twofacets being opposite to each other form a cavity, characterized in thatsaid active layer in the vicinities of the facets is transparent to theemission wavelength, and the surfaces of the first conduction type ofclad layer, active layer and second conduction type of clad layerforming said facets are coated with a passivation layer 14. Now,explanation will be made by using, as an example, a semiconductor laserof the refractive index guided structure wherein the second conductiontype of clad layer consists of the first and second layers and the later(i.e., second) second conduction type of clad layer and current blocklayers together form a current injection region, and the contactresistance with electrode is lowered by a contact layer.

The basic epitaxial structure of such lasers is exemplified by onedescribed in JP-A-8-130344 (Horie, et al.) in which two etching stoplayers are formed on the substrate in contact with a layer comprising atleast Al_(x)Ga_(1−x)As and ridges or grooves are formed by wet etching.These lasers are used as light sources in optical fiber amplifiers foroptical communication light sources for pick-up in large-capacitymagnet-optical memory for data processing. Furthermore, these lasers areapplicable to various uses by varying the constitution of the activelayers and clad layers, using various materials, etc.

FIG. 2 is a model view of the epitaxial structure of a groove typesemiconductor laser of the present invention.

By taking the desired emission wavelength, lattice matching, strainintentionally introduced into the active layer, etc. into consideration,use is made as the substrate (1) of a single crystal substrates of InP,GaAs, GaN, InGaAs, Al₂O₃, etc. Some dielectric substrates such as thosemade of Al₂O₃ are also usable in some cases. In the embodiments of thepresent invention, it is preferable to use an InP substrate or a GaAssubstrate from the viewpoint of the lattice matching with III-V groupcompound semiconductor light emitting devices containing As, P, etc.When As is contained as an element of the V group, it is most desirableto use a GaAs substrate.

Dielectric substrates such as those made of Al₂O₃ are sometimes employedfor the growth of materials containing nitrogen as an element of the Vgroup in III-V group semiconductor light emitting devices.

Use can be made of not only so-called just substrates (orientedsubstrates, for example, (100) substrate)but also so-called off-anglesubstrates (miss oriented substrates, for example 2° off from (100)surface) so as to improve the crystallinity in the epitaxial growth.Because of having an effect of promoting good crystal growth in theso-called step flow mode, these off-angle substrates are employedwidely. Although off-angle substrates are usually inclined by 0.5 to 2°,those inclined around 10° are used in some material systems constitutingthe quantum well structure.

The substrates are sometimes subjected to chemical etching, heattreatment, etc. as a pretreatment for the production of light emittingdevices with the use of the crystal growing techniques such as MBE(Molecular Beam Epitaxy)or MOCVD(Metal Organic Chemical VaporDeposition).

It is preferable to provide a buffer layer (2) to relieve theinsufficiency in the substrate bulk crystals and facilitate theformation of epitaxial films having the same crystal axis. It ispreferable that the buffer layer (2) and the substrate (1) are made ofthe same compound. When a GaAs substrate is used, for example, then GaAsis employed as the buffer layer too in general. However, it has been apractice too to use a superlattice layer as the buffer layer. Namely,the buffer layer is made of a different compound from the one of thesubstrate in some cases. When a dielectric substrate is used, on theother hand, it is not always needed that the buffer layer is made of thesame compound. In such a case, a material different from that of thesubstrate is appropriately selected as the buffer layer depending on thedesired emission wavelength and the structure of the whole device.

The first conduction type of clad layer (3) is usually made of amaterial having a lower refractive index than the average refractiveindex of the active layer (4). This material is appropriately specifieddepending on the substrate (1), buffer layer (2), active layer (4), etc.prepared for establishing the desired emission wavelength. When GaAs isemployed as both of the substrate and buffer layer (2), then AlGaAssystem, InGaAs system, AlGaInP system, InGaP system, etc may be used asthe material for the first conduction type of clad layer. It issometimes possible to make the whole clad layer superlattice structure.

In one feature of the present invention first, it is important that theactive layer in the vicinities of facets transparent to the emissionwavelength, since no light absorption occurs and heating or degradationdue to heating can be preventing thereby. Combination of p-type activelayer and Si-containing passivation layer, which is a feature of thesecond invention, can also prevent light absorption. In general, theband gap of a semiconductor becomes smaller with an increase in the Halldensity thereof, as described in, for example, Heterostructure Lasers(H. C. Casey, Jr. and M. B. Panish, Academic Press 1978, p. 157) In thecase of GaAs, for example, the band gap Eg (eV) is expressed by thefollowing formula:

Eg=1.424−1.6×10⁻⁸ ×P ^(⅓)

wherein P (cm⁻³) means the p type carrier density. One of thecharacteristics of the present invention resides in that Si which is apassivation layer (14) to be inserted between the laser facet and thematerial of the passivation layer, for example, Si is introduced intothe active layer (4), etc. as n-type impurities during long termoperation of the laser, in particular, at high output, thus lowering theeffective Hall density due to the compensation effect. This fact meansthat the vicinities of the semiconductor, i.e., Si broaden the band gapof the part which is to be diffused as the LD is driven. It is expectedthat the light absorption at the facet is thus suppressed. When III-Vgroup compound semiconductors are used as the active layer, it ispreferable to use C, Be, Mg, etc. as the dopant and the carrier densityappropriately ranges from 1×10¹⁴ (cm⁻³) to 1×10¹⁸ (cm⁻³), stillpreferably from 1×10¹⁵ (cm⁻³) to 1×10¹⁷ (cm⁻³).

When the active layer is p-type, an effect of suppressing the lightabsorption at the facet can be achieved by broadening the band gap ofthe active layer facet due to the diffusion of Si to the active layer.

When the active layer is one of the n-type and the undoped, abovementioned mechanism for broadening the band gap as done in the case ofthe p-type does not function. However, even when the active layer is notp-type, if the facet of the active layer is transparent for the emissionwavelength by means explained in the latter part, heating or degradationdue to heating can be effectively prevented thereby.

From the viewpoint of selecting the material, it is preferable that theactive layer (4) is made of a material containing In more preferably Inand Ga. This is because such a system is liable to be ordered during thecrystal growth. Namely, when Si inserted as the passivation layer (14)between the laser facet and Si diffuses when the laser is driven, asdescribed above, it is expected that the vicinities of the facet mightbe disordered. In general, a disordered material results in an increasein the band gap, which together with the compensation effect of thecarrier suppresses further light absorption at the facet for a longtime.

By taking these factors into consideration, it is preferable to useAlGaAs system, InGaAs system, InGaP system, AlGaInP system, etc. for theactive layer (4). Among all, those active layers having the quantum wellstructure are preferable to achieve disordering. These materials areusually selected depending on the desired emission wavelength.

Although the active layer (4) as a usual bulk active layer consisting ofa single layer, quantum well structures such as single quantum well(SQW) structure, double quantum well (DQW) structure, multiple quantumwell (MQW) structure, etc. can be employed depending on the purpose. Anactive layer of the quantum well structure is usually employed togetherwith an optical guide layer. To separate the quantum well, a barrierlayer may be employed, if needed. As the structure of the active layer,use can be made of the separated confinement hetero structure (SCH)wherein optical guide layers are provided in both sides of the quantumwell, grated index-SCH (GRIN-SCH) wherein the refractive index iscontinuously changed by gradually varying the composition of the opticalguide layer, etc. The material of the optical guide layer may beappropriately selected from among AlGaAs system, InGaAs system, InGaPsystem, AlGaInP system, etc. depending on the active layer.

Similar to the first conduction type of clad layer (3), the first andsecond layers (5) and (8) of the second conduction type of clad layerare made of a material having a lower refractive index than the averagerefractive index of the active layer (4). This material is appropriatelyspecified depending on the substrate (1), buffer layer (2), active layer(4) etc. When GaAs is employed as both of the substrate and buffer layer(2), then use may be made of AlGaAs system, InGaAs system, AlGaInPsystem, InGaP system, etc.

FIG. 2 shows two types of etching stop layers (6) and (7) and a caplayer (10). These layers, which are employed in a preferable embodimentof the present invention, are effective in precisely and easily formingthe current injection region.

When the second etching stop layer (6) is made of, for example, an Al₁Ga_(1−a)As (0≦a≦1) material, it is adequate to use GaAs. This is becausethe second layer of the second conduction type of clad layer (8), etc.can be grown with a good crystallinity in the re-growth in the AlGaAssystem. It is usually preferable that the second etching stop layer is 2nm or more in thickness.

As the first etching stop layers (7), it is appropriate to use a layerrepresented by In_(b)Ga_(1−b)P (0≦b≦1). When GaAs is employed as thesubstrate as in the case of the present invention, b=0.5 is employedusually in a lattice-matched system. The first etching stop layer (7) isusually 5 nm or more, preferably 10 nm or more, in thickness. When itsthickness is less than 5 nm, it is feared that etching cannot beinhibited due to uneven film thickness, etc. On the other hand, a strainsystem can be used depending on the film thickness and, in such a case,b may be 0 or 1.

Cap layers (10) are employed as protective layers for the current blocklayers (9) in the first growth and to facilitate the growth of thesecond layer (8) of the second conduction type of clad layer. Beforeobtaining the element structure, these cap layers are partly orcompletely eliminated.

The current block layers (9) should literally block current. Therefore,it is preferable that these layers have the same conduction type of thatof the first conduction type of clad layer (3) or undoped. For example,it is preferable that a current block layer (9) made of an AlGaAs systemhas a refractive index lower than that of the second layer (8) of thesecond conduction type of clad layer made of Al_(y)Ga_(1−y)As (0<y≦1).When the current block layer is Al_(z)Ga_(1−z)As (0≦z≦1), therefore, itis preferable that z is larger than y in the alloy.

The second layer (8) of the second conduction type of clad layer usuallyhas a refractive index lower than that of the active layer (4). Thesecond layer (8) of the second conduction type of clad layer is usuallythe same as the first conduction type of clad layer (3) and the firstlayer (5) of the second conduction type of clad layer. In a preferredembodiment of the present invention, the first layer (5) of the secondconduction type of clad layer, the second layer (8) of the secondconduction type of clad layer and the current block layer (9) are allmade of the same material system of the same composition. In such acase, a difference in effective refractive index is formed by the firstetching stop layers (7). When the cap layers (10) are not completelyeliminated, a difference in effective refractive index is formed by thecap layers (10), in addition to the one formed by the first etching stoplayers (7). This layered structure is highly preferable, since variousproblems caused by the difference in the materials or compositions ateach interface between the second layer (8) of the second conductiontype of clad layer and the current block layer (9).

It is preferable to provide a contact layer (11) on the second layer (8)of the second conduction type of clad layer so as to lower the contactresistance with the electrode (12). This contact layer (11) is usuallymade of a GaAs material. To lower the contact resistance with theelectrode (12), the carrier density in this layer is higher than otherlayers.

In usual, the thickness of the buffer layer (2) ranges from 0.1 to 3 μm;that of the first conduction type of clad layer (3) ranges from 0.5 to 3μm; that of the active layer ranges from 0.0005 to 0.02 μm per layer (inthe case of the quantum well structure); that of the first layer (5) ofthe second conduction type of clad layer ranges from 0.05 to 0.3 μm;that of the second layer (8) of the second conduction type of clad layerranges from 0.5 to 3 μm; that of the cap layer (10) ranges from 0.05 to0.5 μm; and that of the current block layer (9) ranges from 0.3 to 2 μm.

The semiconductor light emitting device as shown in FIG. 2 is furtherprovided with electrodes (12) and (13). In the p-type, the electrode(12) is formed by metallizing the surface of the contact layer (11)with, for example, Ti/Pt/Au successively and then alloying. On the otherhand, the electrode (13) is formed on the surface of the substrate (1).In the n-type, the substrate (1) is successively deposited withAuGeNi/Au followed by alloying.

The wafer of the semiconductor laser thus formed is cleaved so as togive a so-called laser bar. In the present invention, troublesomecleavage in vacuum is not always needed in general. This is because,when cleavage is performed in air at atmospheric pressure or in anitrogen atmosphere, it is possible to eliminate oxide of at least oneof the elements constituting the first conduction type of clad layer(3), active layer (4), second conduction type of clad layers (5) and(8), the substrate (1), buffer layer (2), first etching stop layer (7),second etching stop layer (6), current block layer (9), cap layer (10),contact layer (11), etc, each exposed at the facets acting asnon-radiative recombination center at the facets of the elements in thevicinities of the cavity facets.

Plasma irradiation may be cited as one of the techniques therefor. It isparticularly effective to irradiate with ionized rare gas plasma, stillpreferably Ar plasma. It is preferable to control the irradiation energyof this Ar plasma to a low level of 25 to 300 eV, still preferably 25 to100 eV. Thus oxide(s), nitride (s), etc. acting as non-radiativerecombination center can be eliminated. This treatment is superior tothe conventional ones such as ion-implantation of impurities in that itcan be effected at an extremely low energy and thus the process can becompleted while preventing the facets from damages. From the viewpointof eliminating oxides from the facets, As—O can be thus eliminatedparticularly effectively. Also, Ga—O, etc. can be effectively eliminatedthereby.

In other words, it is essential in the present invention that at leastone of the constituting elements of the first conduction type of cladlayer, active layer and second conduction type of clad layer, which areusually provided even in a semiconductor laser with the simpleststructure, the vicinities of facets do not have their oxides. Asdescribed above, it is important that oxide(s) (As—O, Ga—O, etc.) actingas non-radiative recombination center on the facets are eliminated.

However, it is reasonable in general that oxides of the elements in thevicinities of the facets would be thus eliminated too in other layers ofa laser such as the substrate and the current block layer. This isbecause the whole facets are usually irradiated with the above-mentionedAr plasma. Needless to say, it is important to treat the firstconduction type of clad layer, active layer and second conduction typeof clad layer as described above.

More particularly speaking, it is most important in a laser having therefractive index guided structure, which has been cited above as apreferred embodiment of the present invention, that at least one of theelements constituting the first conduction type of clad layer (3),active layer (4) and second conduction type of clad layers (5) and (8)does not exist as its oxide at the facets thereof. In associationtherewith, the components such as the substrate (1), buffer layer (2),first etching stop layer (7), second etching stop layer (6), currentblock layer (9), cap layer (10), contact layer (11), etc., each exposedat the facets, are also irradiated with the plasma and, as a result,oxides are eliminated therefrom. This phenomenon never inhibits theachievement of the objects of the present invention. In the currentblock layer, etc. wherein some of near-field pattern remain, it israther preferable that no oxides or nitrides of the constitutingelements exist at the facet of the block layer.

Methods for analyzing the presence of the oxide of at least oneconstituting element are exemplified by the X-ray photoelectronspectroscopy (XPS). This method is highly advantageous in analyzing thechemical binding manner of each element. The facets of a laser isirradiated with X-ray of 100 μm×100 μm and then the photoelectrons thusformed are subjected to energy spectroscopy, thus confirming thechemical binding manner of each element constituting the laser facets.By varying the angle of the photoelectron detector to the sample surfacein this step, information in the vicinities of alone can be easilyobtained. As will be described hereinafter, lasers are generally coatedwith a dielectric material optionally combined with a semiconductor atthe facets. Prior to the above-mentioned XPS analysis, it is therefore apractice to reduce the thickness of the coating film so as to give to athickness appropriate for the analysis by various etching techniques. Inthe case of a laser having a thin coating film of about 2 nm, the facetsof the semiconductor laser can be analyzed without effecting the aboveetching procedure, etc.

On the other hand, the effects achieved by the irradiation with lowenergy argon plasma is not only the elimination of oxides such as Ga—O,As—O as described above. When the active layer forms quantum well and ismade of, in particular, a system containing In or more preferably In andGa such as an AlGaAs-system material, an InGaP-system material or anAlGaInP-system material, the vicinities of the facets can be disorderednot due to the diffusion of Si during driving but in the process ofproducing the laser. In this case, moreover, facets with higherresistance can be prepared. These facts indicate in the early stage ofthe formation of the laser, it is possible to broaden the band gap inthe vicinities of the facets and, at the same time, increase theresistance in the vicinities of the facets. Thus, light absorption atthe facets can be suppressed and the current injection into the facetsliable to be broken can be also inhibited, thus further prolonging thelife of the laser.

That is to say, the above-mentioned plasma irradiation is efficacious inbroadening the band gap in the vicinities of the facets during theprocess of fabricating LD. Moreover, the band gap in the vicinities ofthe facets is further broadened when the LD is driven, which makes itpossible to embody an element having high output and high reliability.

Cleavage is appropriately employed in forming the facets. Although thisprocedure is widely used in edge-emission lasers, it is sometimes usablein a case where cavities are formed during the growth of crystals, as invertical cavity surface emitting lasers.

The facet formed by cleavage differs depending on the orientation of thesubstrate employed. To form an element such as an edge-emission laser byusing a substrate having a face crystallographically equivalent tonominally (100) face employed, (110) face or a face crystallographicallyequivalent thereto serves as each of the faces for forming a cavity.When the above-mentioned off-angle substrate (miss oriented substrate)is employed, the facets do not always meet at right angles with thedirection of the cavity depending on the inclination direction. When asubstrate with an inclination of 2° to the direction (1-10) to thesubstrate (100) is employed, each of the facets is also inclined by 2°.

In the present invention, a passivation layer is a layer deposited onthe facet of the semiconductor light emitting device, preventing thefacet from degradation due to chemical reaction with oxygen.

The passivation layer should be formed to cover at least the firstconduction type of clad layer, the active layer, and the secondconduction type of clad layer forming the facet. However, thepassivation layer is usually formed to cover the whole surface of thefacet.

Si, Ge, S and Se are suitable materials for the passivation layer, butSi is thought to be the best. It is preferable that the passivationlayer contains more than 50 atomic % of Si.

Although Si deposited to the semiconductor facet as the passivationlayer (14) as shown in FIG. 2 has various crystallographiccharacteristics, either single crystal, poly-crystalline or amorphouscan achieve the effects.

It is particularly appropriate to use amorphous Si formed at a lowdeposition rate in high vacuum. Although the band edge of Si variesdepending on the film properties in general, it is transparent to lightof wavelength of about 2 μm or longer without showing any absorption. Onthe contrary, Si shows a refractive index N of (n+ik) to light ofemission wavelength of about 2 μm or shorter, wherein n means the realnumber part of the refractive index, k is an extinction coefficient andn is about 3.5.

It is generally preferable that the thickness of the passivation layer(14) is thicker than 0.2 nm. However, an extremely thick passivationlayer of, for example, 100 nm is unsuitable in some cases. The lowerlimit of the preferable thickness of the passivation layer (14) isspecified by the factors for the existence of the passivation layer perse as a film, while the upper limit thereof is determined from thebalance with the effect of Si of absorbing light emitted from the activelayer. When Si is deposited on the facet, namely, consideration shouldbe made on the factor of coating the whole facet with the passivationlayer and the effect of an increase in the temperature at the facet dueto the absorption by Si. According to the experiments carried out by thepresent inventors, the following range is preferable:

0.2 (nm)<T _(Si)<λ/8n (nm)  (I)

wherein n means the real number part of the refractive index of theabove-mentioned Si layer at emission wavelength λ.

However, the effects are confirmed when the passivation layer has athickness not more than 0.2 nm.

In general, a semiconductor laser is preferably provided with a coatinglayers (15) and (16) comprising a dielectric material passivationoptionally combined with a semiconductor and deposited on thepassivation layers (14) formed on the exposed semiconductor facets. Itis still preferable that the ion-irradiation of the facets, theformation of the passivation layers (14) and the formation of thecoating layers (15) and (16) are successively performed in vacuumwithout braking vacuum to thereby increase an external differentialquantum efficiency and further protect the facets. To achieve a highoutput, it is a practice to form an anti-reflective coating layer on thefront facet and a high-reflective coating layer on the rear facet, i.e.,asymmetric coating.

Various materials are usable in this coating method. It is preferable touse one or more compounds selected from the group consisting of AlO_(x),Tio_(x), SiO_(x), SiN, Si and ZnS. As the anti-reflective coating layer,use is made of AlO_(x), Tio_(x), SiO_(x), etc., while use is made of anAlO_(x)/Si multilayered film, a Tio_(x)/SiO_(x) multilayered film, etc.for the high-reflective coating layer.

The thickness of each layer is controlled so as to give the desiredreflectance. It is a practice to use AlO_(x), Tio_(x), SiO_(x), etc. asthe anti-reflective coating layer while controlling so as to make thefilm thickness at emission wavelength λ about λ/4n, wherein n means thereal number part of the refractive index. Also, the high-reflectivecoating layer made of various materials is controlled to give a filmthickness of about λ/4n. It is appropriate to further deposit a pair ofthese layers depending on the purpose.

To form the coating layer (15) and (16), it is adequate to use theso-called ion assisted deposition (IAD) method. In this method whichcomprises evaporation of the coating materials simultaneously withirradiation with ion of a definite energy, it is particularly preferableto perform ion deposition with a rare gas. IAD with Ar ion, among raregases, is particularly effective in improving the film properties of theabove-mentioned coating materials. It is still preferable to use Ar ionwithin a low energy range of from 25 to 300 eV, still preferably 50 to200 eV. Thus coating can be completed without damaging the semiconductorfacets.

In the third feature of the present invention, a wafer obtained byforming electrodes is cleaved into laser bars and then at least one offacets forming the cavity is irradiated in vacuum with plasma having anoptimized energy. According to the present invention, the facet(s) aretreated after the formation of the electrodes, which makes it possibleto form electrodes on wafer before cleavage for exposing the facets. Asa result, the troublesome step of forming electrodes on each of thelaser bars after cleaving can be omitted, which brings about anindustrial advantage compared with the conventional process wherein thefacets are made transparent by forming a semiconductor layer thereon.

As the plasma to be irradiated, it is preferable to use those of theelements of the group 18 (Ar, etc.).

The Ar plasma to be irradiated has an energy of from 25 eV to 300 eV.When the energy of this Ar plasma is less than 25 eV, the irradiationwith the Ar plasma can achieve only an insufficient effect and thus theproblem of the sudden failure during long term operation of the laserunder APC mode cannot be solved thereby. When the energy exceeds 300 eV,on the other hand, disordering arises too vigorously and thus the laseris liable to be broken, which results in a decrease in the maximumoutput in the initial characteristics. In this step, it is preferable toperform the plasma irradiation at a current density of from 1 μA/cm² to1 mA/cm² for 15 seconds to 30 minutes.

Subsequently, the facets may be asymmetrically coated. Usually, asingle-layered AlO_(x) film, SiO_(x) film, SiN_(x) film, etc. is formedon the front facet from which light is taken out to give a lowreflective face, while a multi-layered AlO_(x)/α-Si film,SiO_(x)/Tio_(x) film, etc. is formed on the rear facet to give a highreflective face. In this step, it is preferable that the reflectance inthe front facet side ranges from 0.5 to 20%, still preferably from 1 to10%, while that in the rear facet side ranges from 50 to 98%, stillpreferably from 85 to 95%. In the present invention, the asymmetriccoating of the facets is preferably effected following the plasmairradiation under continuous evacuation. The laser bars asymmetricallycoated at the facets are divided into chips and employed as laser diodes(LD).

FIG. 24 is a perspective view of the semiconductor laser of the presentinvention. The facets and vicinities thereof (region 25) are thehigh-resistant regions formed by the plasma irradiation and quantumwells in the vicinities of the facets are disordered thereby. Thus, thevicinities of the facets are made transparent to the emission wavelengthof the laser. In the shaded parts, carriers are passivation and madehighly resistant by exposing each layer to the plasma. Thus, current canhardly pass therethrough and no current is injected into both facets,which lessens degradation in the characteristics of the element due toreactive current, etc.

The semiconductor laser of the present invention is one wherein a firstconduction type of clad layer, an active layer containing quantum welland a second conduction type of clad layer are provided on a substrate,characterized in that at least one of the facets forming the cavity isirradiated from the facet side with plasma having energy of from 25 eVto 300 eV. Thus, highly resistant regions are formed in the vicinitiesof the facets of the semiconductor substrate, first conduction type ofclad layer, active layer containing quantum well and second conductiontype of clad layer. Such a region shows a higher resistance than theparts in the same layer less affected by the plasma irradiation. Thehighly resistant region in the vicinities of the facet of the activelayer containing quantum well is disordered. Such a semiconductor lasermakes it possible to very easily and stably produce the so-called windowstructure. Since the higher resistant regions are formed at the sametime, moreover, there arises no problem of the so-called reactivecurrent. Thus semiconductor laser diodes having high output and highreliability can be easily obtained.

Although the above description relates to semiconductor lasers of thegroup having the refractive index guided structure mechanism, thepresent invention is applicable to any semiconductor lasers (ridge typesemiconductor lasers, lasers having the gain guided structure, etc.)regardless of the constitution thereof, so long as these lasers have thecharacteristics as set forth in the claims of the present application.

The process for producing a semiconductor light emitting device which isthe fourth feature of the present invention, is one for producing asemiconductor light emitting device having a compound semiconductorlayer containing a first conduction type of clad layer, an active layerand a second conduction type of clad layer and formed on a substrate andhaving a cavity, characterized by comprising forming the compoundsemiconductor layer on the substrate by the successive crystal growth;next forming the cavity facets; then desorping (removing) at least apart of the constituting elements in the vicinities of the active layer;and forming passivation layers in vacuum. The structure of the element,etc. are not particularly restricted, so long as the process has thecharacteristics described above. Now, illustration will be made, by wayof example, on the application of the present invention to theproduction of a semiconductor laser having the refractive index guidedstructure, wherein the second conduction type of clad layer is dividedinto the first and second layers, a current injection region is formedby the second layer of the second conduction type of clad layer and thecurrent block layer, and contact layers are provided to lower thecontact resistance with electrodes.

Similar to first to third features as described above, the thus formedsemiconductor wafer is cleaved to give so-called laser bars. In thepresent invention, troublesome cleavage in vacuum is not always neededin general. This is because, when cleavage is performed underatmospheric pressure or in a nitrogen atmosphere, it is possible tosuppress the absorption of the emission wavelength at the facets. One ofthe techniques therefor is to desorp a part of the constituting elementsin the vicinities of the facet of the active layer to thereby make theactive layer in the vicinities of the facet transparent to the emissionwavelength. Particular procedures therefor include irradiation of anfacet with heat beam with quick heat response, irradiation with chargedparticles (ion beam, electron beam or plasma which is a combination ofion and electron), photo irradiation, etc.

As described above, the material of the active layer (4) may beappropriately selected depending on the desired emission wavelength andoutput. In the case of In_(u)Ga_(1−u)As (0.15≦u≦0.35) usually employedto give an emission wavelength of about 980 nm, the vapor pressuresrelating to the desorption of the As compounds in vacuum are expressedas InAs>GaAs>AlAs. From these mixed crystal semiconductors, a certainelement can be selectively desorped by selecting an appropriate treatingtemperature. For example, InAs in InGaAs is selectively desorped at 500to 650° C. Thus, a region with a low In density can be formed in theheat treated region by selectively heating the vicinities of the facet.Thus, the band gap in the vicinities of the surface can be broadened anda so-called window structure can be formed exclusively in the vicinitiesof the facet.

The heat source or light source to be used for irradiation may be anyone, so long as it contains the wavelength absorbed by the materialemployed for the active layer. Usually, a halogen lamp, a xenon lamp,etc. may be appropriately employed as a heat source or a light source.

In the case of electron-irradiation, it is preferable to use electronbeam of 100 eV to 100 KeV in energy. The irradiation dose may beappropriately controlled so that the surface temperature of the laserbars is increased to the desired level. To relieve the heat load of thewhole laser bars, it is preferable to complete the irradiation within ashort period of time, namely, not longer than 10 minutes in usual,preferably not longer than 5 minutes.

In the fifth feature of the present invention, the wafer thus completedafter the formation of the electrodes is cleaved into laser bars andthen at least one of the facets forming the cavity is irradiated invacuum with ion, electron, light and/or heat. In the present invention,the facets can be treated after the formation of the electrodes. Inother words, the electrodes can be formed on the wafer before cleavagefor exposing the facets. As a result, the troublesome step of formingelectrodes on each of the laser bars after cleavage can be omitted,which brings about a structural advantage compared with the conventionalprocess wherein the facets are made transparent by forming asemiconductor layer thereon.

The ion, electron light and/or heat to be irradiated may beappropriately selected depending on the purpose of selectively desorpingelement(s) with high vapor pressure. In the case of phtoirradiation, thelight source may be any one, so long as it contains the wavelengthabsorbed by the material employed for the active layer. Usually, ahalogen lamp, etc. may be appropriately employed therefor. In the caseof electron-irradiation, it is preferable to use electron beam of 100 eVto 100 KeV in energy. The irradiation dose may be appropriatelycontrolled so that the surface temperature of the laser bars isincreased to the desired level. To relieve the heat load of the wholelaser bars, it is preferable to complete the irradiation within a shortperiod of time, namely, not longer than 10 minutes in usual, preferablynot longer than 5 minutes. These heating conditions may be determined bypreparing samples for measuring temperature together with the laser barsand estimating the temperature increased after the completion of theprocess. However, the effect of transparent finishing cannot be achievedat a low temperature, while an excessively high temperature causestroubles in chips. Thus, the optimum heating conditions can beestablished by repeating the experiment several times.

Thus, impurities such as oxides remaining on the facets can beeliminated by heating the vicinities of the facet via irradiation withion, electron, light and/or heat. At the same time, specific element(s)are selectively desorped from the region in the vicinities of the facetsby taking advantage of the difference among desorption rate of theelements constituting the active layer. Thus a region having a broaderband gap than the energy corresponding to the emission wavelength of thelaser can be efficiently formed in the vicinities of the facet of theactive layer.

Subsequently, the facets may be asymmetrically coated. Usually, asingle-layered AlO_(x) film, SiO_(x) film, SiN_(x) film, etc. is formedon the front facet from which light is taken out to give a lowreflection, while a multi-layered AlO_(x)/α-Si film, SiO_(x)/Tio_(x)film, etc. is formed on the rear facet to give a high reflection. In thepresent invention, the asymmetric coating of the facets is preferablyeffected following the plasma irradiation under continuous evacuationwithout breaking a vacuum after Ar irradiation. The laser barsasymmetrically coated at the facets are divided into chips and employedas laser diodes (LD). FIG. 25 is a perspective view of the semiconductorlaser of one of the present invention. The facets and vicinities thereofare the transparent regions 35 suffering from the selective desorptiondue to the irradiation with ion, electron, light and/or heat electronbeam. The vicinities of both facets are made transparent to the emissionwavelength of the laser. The semiconductor laser of the presentinvention comprises having a first conduction type of clad layer, anactive layer containing quantum well and a second conduction type ofclad layer on a semiconductor substrate, wherein at least one of thefacets forming the cavity is irradiated with optimized ion, electron,heat and/or light to thereby form a region with different compositiondue to desorption at least in the vicinities of the facet of the activelayer containing quantum well. Namely, the semiconductor laser of thepresent invention is characterized in that said region has a broaderband gap than that of the adjacent unirradiated regions in the samelayer and is transparent to the emission wavelength of the laser. Thisso-called window structure can be very conveniently and stablyconstructed. At the same time, moreover, impurities remaining on thefacets can be eliminated and thus the surface state density due to theseimpurities can be lowered. Thus, semiconductor laser diodes having highoutput and high reliability can be easily obtained.

The structure of the semiconductor light emitting device of the sixthfeature of the present invention is nor particularly restricted, so longas it is a compound semiconductor light emitting device having anemission wavelength of λ (nm) wherein at least a first conduction typeof clad layer, an active layer and a second conduction type of cladlayer are grown on a substrate and two facets being opposite to eachother form a cavity, characterized in that the surfaces of the firstconduction type of clad layer, active layer and second conduction typeof clad layer forming the facets are transparent, in the vicinities ofthe active layer, to the emission wavelength λ (nm) and coated withpassivation layers comprising silicon and the surfaces of thesepassivation layers are coated with coating layers comprising adielectric material optionally combined with a semiconductor. As aparticular example of this structure, illustration will be made on asemiconductor laser having the refractive index guided structure,wherein the second conduction type of clad layer is divided into thefirst and second layers, a current injection region is formed by thesecond layer of the second conduction type of clad layer and the currentblock layer and a contact layer is further provided so as to lower thecontact resistance with the electrode.

To further illustrate the present invention in greater detail, thefollowing Examples will be given. However, it is to be understood thatthe present invention is not restricted and various modification may bemade within the true spirit and scope of the invention.

EXAMPLE 1

Now, the first example of the present invention will be described.

In this example, facets of a cavity of a semiconductor laser wereirradiated with argon plasma in vacuum. Then amorphous silicon waselectron beam-evaporated onto these facet in vacuum to give passivationlayer 14. Next, AlO_(x)layers were formed respectively on the front andrear facets by the IAD (Ion Assisted Deposition)method to give coatinglayers 15 and 16 (refer to FIG. 1).

As FIG. 2 shows, semiconductor layers were successively grown on asubstrate by epitaxial growth and thus a wafer for forming a laser ofthe groove type was formed as shown below.

On an n-GaAs substrate (1) having a carrier density of 1×10¹⁸ cm⁻³ weregrown by the MBE method, an n-GaAs layer having a thickness of 1 μm anda carrier density of 1×10¹⁸ cm⁻³ as a buffer layer (2) and ann-Al_(0.35)Ga_(0.65)As layer having a thickness of 1.5 μm and a carrierdensity of 1×10¹⁸ cm⁻³ as a first conduction type of clad layer (3).Next, on the first conduction type of clad layer (3), an active layer(4) consisting of an undoped GaAs optical guide layer having a thicknessof 24 nm; an undoped In_(0.2)Ga_(0.8)As single quantum well (SQW) havinga thickness of 6 nm; and an undoped GaAs optical guide layer having athickness of 24 nm, which were successively grown and; ap-Al_(0.35)Ga_(0.65)As layer having a thickness of 0.1 μm and a carrierdensity of 1×10¹⁸ cm⁻³ as a first layer (5) of the second conductiontype of clad layer; a p-GaAs layer having a thickness of 10 nm and acarrier density of 1×10¹⁸ cm⁻³ as a second etching stop layer (6); ann-In_(0.5)Ga_(0.5)P layer having a thickness of 20 nm and a carrierdensity of 5×10¹⁷ cm⁻³ as a first etching stop layer (7); ann-Al_(0.39)Ga_(0.61)As layer having a thickness of 0.5 μm and a carrierdensity of 5×10¹⁷ cm⁻³ as a current block layer (9); and an n-GaAs layerhaving a thickness of 10 nm and a carrier density of 1×10¹⁸ cm⁻³ as acap layer (10) were grown by the MBE method successively.

Next, all the surface of the uppermost layer except the currentinjection region was covered with a SiN_(x) mask having an window of 1.5μm in width. Etching was performed by using the first etching stop layeras an etching stop layer so as to eliminate the cap layer (10) and thecurrent block layer (9) in the current injection region (correspondingto a region exposed by the window) This etching was carried out with theuse of a mixture of sulfuric acid (98 wt. %), hydrogen peroxide (a 30%aqueous solution) and water (1:1:5 by volume) as an etchant at 25° C.for 30 seconds.

Then the SiN_(x) layer was eliminated by immersing the substrate withcompound semiconductor layers in an etching solution comprising HF (49%)and NH₄F (40%) (1:6) for 2.5 minutes. Subsequently, the first etchingstop layer in the current injection region was eliminated by etchingwith the use of the second etching stop layer as an etching stop layer.This etching was carried out with the use of a mixture of hydrochloricacid (35 wt. %) and water (2:1) as an etchant at 25° C. for 2 minutes.

Subsequently, a p-Al_(0.35)Ga_(0.65)AS layer having a carrier density of1×10¹⁸ cm⁻³ was grown as the second layer (8) of the second conductiontype of clad layer by MOCVD method to give a thickness of 1.5 μm inburied part (i.e., the current injection region). Finally, a p-GaAslayer having a thickness of 7 μm and a carrier density of 1×10¹⁹ cm⁻³was grown as a contact layer (11) to realize good electric contact withthe electrode to thereby form a laser element. The width W of thecurrent injection region, i.e., the interface of the second layer of thesecond conduction type of clad layer with the second etching stop layerwas 2.2 μm.

Onto this wafer, AuGeNi/Au was evaporated on the substrate side as ann-type electrode, while Ti/Pt/Au was evaporated as a p-type electrodefollowed by alloying at 400° C. for 5 minutes to thereby form a wafercompletely.

Next, this wafer was cleaved into laser bars of 700 μm in cavity lengthand these laser bars were put into a vacuum chamber with an ion-gun togenerate Ar plasma. Then the front facet was irradiated from the side ofthis facet with Ar plasma of 60 eV in average energy and 150 μA/cm² incurrent density for 1 minute. Subsequently, amorphous Si was depositedin a thickness of 2 nm on the front facet by the conventional electronbeam evaporation method. Then an AlO_(x) film of 165 nm in thickness wasformed as a coating layer so as to make the reflectance at the frontfacet 2.5% at an emission wavelength of 980 nm. In the AlO_(x) filmformation, Ar plasma having average energy of 120 eV and current densityof 200 μA/cm² and source of AlO_(x) were applied to the front facet atthe same time, which is so-called IAD method. It was confirmed that thereal number part of the refractive index of the amorphous Si at 980 nmwas about 3.4.

Subsequently, the laser bars were once taken out from the vacuum chamberto treat the rear facet. Then the rear facet was treated in the samemanner as those employed for the front facet except that the coatinglayer was formed so as to consist of four films, namely, 170 nmAlO_(x)/60 nm Si/170 nm AlO_(x)/60 nm Si. As a result, the reflectanceat the rear facet was 92%. Also, the AlO_(x) films were formed by IADmethod in the same manner as those employed for the front facet.

From these laser bars, 10 devices were put onto a heat sink and packagedin a nitrogen atmosphere. These devices showed a threshold current of 23mA at 25° C. and kink was observed at 350 mA, 250 mW as the averageinitial characteristics. When subjected to an accelerated life test (200mW, 50° C.) of APC (Automatic Power Control) mode, these samples showedno sudden failure within 2,000 hours, as shown in FIG. 3, indicatingstable driving. Neither Ga—O nor As—O is detected when such treatedfacets are subjected to XPS measurement with the detection angle ofphoto-electron of 75°. Ga—O and As—O exist on the Gas (110) face onceexposed to the atmosphere.

Further, one of the elements thus formed is processed into a sample fortransmission electron microscopic observation and the active layer inthe vicinities of the facets are compared with bulks. As a result, it isconfirmed that the vicinities of the facets irradiated with the Arplasma show damaged crystallinity, i.e., disordering.

One device is taken out from these laser bars as a sample for analysis.The AlO_(x) layer and Si layer at the front facet are eliminated byusing a hydrofluoric acid-based etchant. Then the device is introducedinto an apparatus for vacuum analysis and the band gap in the vicinitiesof the front facet of the active layer is measured by the electronenergy-loss spectroscopy. This electron energy-loss spectroscopy, bywhich the information exclusively in the vicinities of the samplesurface (the maximum analytical depth: about 1.5 nm) can be obtained, isa useful means for measuring the band gap in the vicinities of the laserfacet without being affected by the physical values of the bulk region.The vicinities of the active layers of laser facets are irradiated withelectron beam (100 nm, 1,000 eV). Then analysis is made on thediffraction energy of loss electron below the surface oxidation layer ata depth of 1 nm. Thus it is found out that the band gap at the InGaAsactive layer facet is 1.5 eV and the band gap on the facet of the GaAsoptical guide layer is 1.65 eV, each due to the loss peak caused byband-to-band transition. The energy gap between quantum levels in theInGaAs quantum well active layer at room temperature, determined bymeasuring the photoluminescence, is 1.29 eV, while the band gap of GaAswas 1.41 eV. Thus it is confirmed that the band gap in the vicinities ofthe facet is broadened mainly by the Ar plasma irradiation and that thefacet is transparent to the emission wavelength.

EXAMPLE 2

Laser bars were produced by the same method as the one of the aboveExample 1 but the coating layer on the rear facet consisted of sixcontinuous films SiO_(x) (200 nm)/TiO_(x) (120 nm)/SiO_(x) (200nm)/TiO_(x) (120 nm)/SiO_(x) (200 nm)/TiO_(x) (120 nm) and to give thereflectance of the rear facet of 88%.

Five devices were put onto a heat sink and packaged in a nitrogenatmosphere. These devices showed a threshold current of 25 mA at 25° C.and kink was observed at 359 mA, 240 mW as the average initialcharacteristics. When subjected to a life test of APC mode (200 mW, 50°C.), these samples showed no sudden failure within 2,000 hours, as shownin FIG. 4, indicating stable driving.

Neither Ga—O nor As—O is detected when such treated facets are subjectedto XPS measurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microscopic observation and the active layer inthe vicinities of the facets were compared with bulks. As a result, itis confirmed that the vicinities of the facets irradiated with the Arplasma show damaged crystallinity, i.e., disordering.

It is confirmed that the band gap in the vicinities of the facet isbroadened mainly by the Ar plasma irradiation and that the facet istransparent to the emission wave length in the same manner as in Example1.

Comparative Example 1

The procedure of Example 1 was repeated but the step of the formation ofthe Si passivation layers on the front and rear facets, and the previousstep of the Ar plasma irradiation were omitted and the conventionalelectron beam evaporation method was employed for all layers as asubstitute for the IAD method. The obtained devices showed a thresholdcurrent of 23 mA at 25° C. and kink was observed at 350 mA, 250 mW asthe average initial characteristics, similar to Example 1. When 10devices were subjected to a life test of APC mode (200 mW, 50° C.), allof the samples underwent sudden failure within 100 hours, as shown inFIG. 5.

Both Ga—O and AS—O are detected when the facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 2

The procedure of Example 1 was repeated but the step of the Ar plasmairradiation, prior to the formation of the Si passivation layers on thefront and rear facets, was omitted. The obtained devices showed athreshold current of 23 mA at 25° C. and kink was observed at 350 mA,250 mW as the average initial characteristics, similar to Example 1.When 10 devices were subjected to a life test of APC mode (200 mW, 50°C.), all of the samples underwent sudden failure within 250 hours, asshown in FIG. 6. Both Ga—O and AS—O are detected when the facets aresubjected to XPS measurement with the detection angle of photo-electronof 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 3

The procedure of Example 2 was repeated but the step of the formation ofthe Si passivation layers on the front and rear facets and the previousstep of the Ar plasma irradiation were omitted. When five devices thusobtained were subjected to a life test of APC mode (200 mV, 50° C.), allof the samples underwent sudden failure within 100 hours, as shown inFIG. 7.

Both Ga—O and As—O are detected when the facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

EXAMPLE 3

Now the third example of the present invention will be described.

Characteristics in this example is that the conduction type of activelayer is p-type.

Laser bars were produced by the same method as the one described inExample 1 except that the active layer(4) grown on the first conductiontype of clad layer (3) is consisting of an n-GaAs optical guide layerhaving a thickness of 24 nm and a carrier density of 5×10¹⁶ cm⁻³; anp-In_(0.2)Ga_(0.8)As single quantum well (SQW) having a thickness of 6nm and a carrier density of 5×10¹⁶ cm⁻³; and a p-GaAs optical guidelayer having a thickness of 24 nm and a carrier density of 5×10¹⁶ cm⁻³.

Then the facets of the laser bars were treated in the same manner asemployed in Example 1 except that the average energy of the Ar plasmaapplied before the formation of Si passivation layer was 90 eV and thatthe coating layer formed on both facets was a single layer of AlO_(x)having a thickness of 2 nm. As a result, the reflectance at the bothfacets was 32% at an emission wavelength of 980 nm.

From these laser bars, 10 devices were put onto a heat sink and packagedin a nitrogen atmosphere. These devices showed a threshold current of 22mA at 25° C. as the average initial characteristics. The optical outputfrom one facet was controlled to a constant output (100 mW) and a lifetest of APC (Automatic Power Control) mode was carried out at 50° C. Asa result, no sudden failure was observed within 2,000 hours, as shown inFIG. 8, indicating stable driving.

One of these laser bars was subjected to XPS measurement as a sample foranalyzing the facets. In this step, the detection angle ofphoto-electron was set to 75° and the conditions of the semiconductorlaser facets were observed. As a result, neither Ga—O nor As—O wasdetected.

Further, one of the elements thus formed is processed into a sample fortransmission electron microscopic observation and the active layer inthe vicinities of the facets are compared with bulks. As a result, it isconfirmed that the vicinities of the facets irradiated with the Arplasma show damaged crystallinity, i.e., disordering.

When measured by electron energy-loss spectroscopy, it is confirmed thatthe band gap in the vicinities of the facet is broadened and that thefacet is transparent to the emission wavelength.

EXAMPLE 4

Laser bars were produced by the same method as the one described inExample 3 but altering the procedure for forming the coating layer onthe rear facet as follows. Then the front and rear facets wereprocessed. Namely, an AlO_(x) film of 165 nm in thickness was formed onthe front facet so as to give a refractive index of 2.5%, while fourcontinuous layers consisting of an AlO_(x) film (170 nm)/an amorphous Silayer (60 nm) /an AlO_(x) film (170 nm)/an amorphous Si layer (60 nm)were formed on the rear facet, by the IAD method for AlO_(x) film and byelectron beam evaporation for Si layer respectively.

From these laser bars, 10 devices were put onto a heat sink(submount)and packaged in a nitrogen atmosphere. These devices showed a thresholdcurrent of 22 mA at 25° C. and kink was observed at 370 mA, 265 mW asthe average initial characteristics. These samples were subjected to anaccelerated life test of APC mode at a constant level (250 mW) at 50° C.As a result, no sudden failure was observed within 2,000 hours, as shownin FIG. 9, indicating stable driving.

Neither Ga—O nor As—O is detected when such treated facets are subjectedto XPS measurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microscopic observation and the active layer inthe vicinities of the facets were compared with bulks. As a result, itis confirmed that the vicinities of the facets irradiated with the Arplasma show damaged crystallinity, i.e., disordering.

When measured by electron energy-loss spectroscopy, it is confirmed thatthe band gap in the vicinities of the facet is broadened and that thefacet is transparent to the emission wavelength.

Comparative Example 4

The same treatments as those described in Example 3 were performed butno Ar plasma irradiation was effected.

From these laser bars, 10 devices were put onto a heat sink and packagedin a nitrogen atmosphere. These devices showed a threshold current of 22mA at 25° C. as the average initial characteristics, showing nodifference from the data of Example 3. The optical output from one facetwas controlled to a constant level (100 mW) and a life test of APC modewas carried out at 50° C. As a result, all devices were damaged within250 hours as shown in FIG. 10.

Both Ga—O and AS—O are detected when the facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

EXAMPLE 5

Now the fifth example of the present invention will be illustrated.

Characteristics in this example reside in that the facets forming thecavity and being opposite to each other were irradiated with plasmahaving energy of from 25 eV to 300 eV.

Thus this example realizes a compound semiconductor light emittingdevice with improvement reliability even when the facets are not coveredwith passivation layers.

Similar to the Examples 3 and 4 described above, a compoundsemiconductor layer structure was first formed by epitaxial growth.Although the active layers in Example 3 were those having a singlequantum well structure of p-type quantum well layers, undoped activelayer having a double quantum well structure were employed herein. Thatis to say, epitaxial growth layers were formed by the same method asthose employed in Examples 3 and 4 but using active layers consisting ofan undoped GaAs optical guide layer (24 nm), an undopedIn_(0.2)Ga_(0.8)As quantum well layer(6 nm), an undoped GaAs barrierlayer (10 nm), an undoped In_(0.2)Ga_(0.8)As quantum well layer (6 nm),and an undoped GaAs optical guide layer (24 nm) to give a wafer forforming a semiconductor laser.

After forming electrodes, the wafer was cleaved into laser bars. Next,both facets were irradiated from the sides of the facets, with Ar plasmaof 100 eV in average energy and 20 μA/cm² in ion current density eachfor 3 minutes. Then a single AlO_(x) layer was formed on the front facetas an anti-reflective coating layer (15) while four layers ofAlO_(x)/amorphous Si/AlO_(x)/amorphous Si were formed on the rear facetas a high-reflective coating layer, thus effecting asymmetric coating of5%/90% (refer to FIG. 1). FIG. 11 shows the initial current opticaloutput characteristics of the element thus obtained. The thresholdcurrent at 25° C. was 21 mA. FIG. 12 shows the results of the life test(200 mW output, 70° C., in the APC mode).

As—O is not detected when such treated facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microscopic observation and the active layer inthe vicinities of the facets are compared with bulks. As a result, it isconfirmed that the vicinities of the facets irradiated with the Arplasma show damaged crystallinity, i.e., disordering.

When measured by electron energy-loss spectroscopy, it is confirmed thatthe band gap in the vicinities of the facet is broadened and that thefacet is transparent to the emission wavelength.

Comparative Example 5

The procedure of Example 5 was repeated but no Ar plasma irradiation wasperformed. FIG. 13 shows the initial current vs optical outputcharacteristics of the element thus obtained. The threshold current at25° C. was 21 mA. FIG. 14 shows the results of the life test (200 mWoutput, 70° C., in the APC mode). Both Ga—O and AS—O are detected whenthe facets are subjected to XPS measurement with the detection angle ofphoto-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 6

The procedure of Example 5 was repeated but using Ar plasma of 20 eV inenergy. FIG. 15 shows the initial current vs optical outputcharacteristics of the element thus obtained. The threshold current at25° C. was 21 mA. FIG. 16 shows the results of the life test (200 mWoutput, 70° C., in the APC mode).

Both Ga—O and AS—O are detected when the facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Further, one of the elements thus formed is processed into a sample fortransmission electron microsopic observation and the active layer in thevicinities of the facets are compared with bulks. As a result, thevicinities of the facets look almost the same as the bulks.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 7

The procedure of Example 5 was repeated but using Ar plasma of 600 eV inenergy. FIG. 17 shows the initial current vs optical outputcharacteristics of the element thus obtained. The threshold current at25° C. was 23 mA. As FIG. 17 shows, no life test could be performedsince the initial output of 200 mW could not be established.

EXAMPLE 6

Now the sixth example of the present invention will be illustrated.

Characteristics in this example reside in that, a part of theconstituting elements of the active layer in the vicinities of the facetwas eliminated before passivation layers were formed in vacuum (refer toFIGS. 1 and 2).

Epitaxial growth layers were formed in the same manner as those ofExample 1, except that the active layer was consisting of an undopedGaAs optical guide layer (30 nm)/undoped In_(0.2)Ga_(0.8)As quantum welllayer (6 nm)/an undoped GaAs optical guide layer (30 nm) to thereby givea wafer for forming semiconductor lasers.

The wafer for semiconductor lasers was formed in the same means as thosedescribed in the above Example 1.

Next, this wafer was cleaved into laser bars of 700 μm in cavity lengthand these laser bars were put into a vacuum chamber with an ion-gun togenerate Ar plasma. Then the front facet was first irradiated from theside of this facet with heat beam and light by using a halogen lamp (4.5KW) for 3 minutes. In this step, the laser bar facet temperature wasabout 550° C. Further, amorphous Si was deposited (2 nm) on the facetand an AlO_(x) film of 165 nm in thickness was formed thereafter so thatthe reflectance of the front facet at emission wavelength of 980 nmbecame 2.5%. In the step of the AlO_(x) film formation, Ar plasma havingaverage energy of 120 eV and current density of 200 μA/cm² and source ofAlO_(x) were supplied to the facet at the same time (IAD method).

Subsequently, the laser bars were once taken out from the vacuum chamberto treat the rear facet. Then the rear facet was treated by irradiatingfrom the side of the facet with heat beam and light by using a halogenlamp (4.5 KW) for 3 minutes. Further, amorphous Si was deposited (2 nm)on the facet and four continuous layers consisting of AlO_(x) film (170nm)/amorphous Si (60 nm)/AlO_(x) film (170 nm)/amorphous Si (60 nm) wereformed to give a rear facet having reflectance of 92%.

From these laser bars, 10 devices were put onto a heat sink and packagedin a nitrogen atmosphere. These devices showed a threshold current of 21mA at 25° C. and kink was observed at 350 mA, 250 mW as the averageinitial characteristics. The emission wavelength of these devices was980 nm. When subjected to an accelerated life test of APC mode (150 mW,50° C.), these samples showed no sudden failure within 1,000 hours, asshown in FIG. 18, indicating stable driving.

Neither Ga—O nor As—O is detected when such treated facets are subjectedto XPS measurement with the detection angle of photo-electron of 75°.Ga—O and As—O exist on the GaAs (100) face once exposed to theatmosphere.

One device is taken out from these laser bars as a sample for analysis.After removing the AlO_(x) layer and Si layer at the front facet byusing a hydrofluoric acid-based etchant, the device is introduced intoan apparatus for vacuum analysis and the band gap in the vicinities ofthe front facet of the active layer is measured by the electronenergy-loss spectroscopy. This electron energy-loss spectroscopy, bywhich the information exclusively in the vicinities of the samplesurface (the maximum analytical depth: about 1.5 nm) can be obtained, isa useful means for measuring the band gap in the vicinities of the laserfacet without being affected by the physical values of the bulk region.The vicinities of the active layers of laser facets are irradiated withelectron beam (beam diameter:100 nm, power:1,000 eV). Then analysis ismade on the diffraction energy of loss electron below the surfaceoxidation layer at a depth of 1 nm. Thus it is found out that the bandgap at the InGaAs active layer facet is 1.45 eV due to the loss peakcaused by band to band transition. The energy gap between quantum levelsin the InGaAs quantum well active layer at room temperature, determinedby measuring the photoluminescence, is 1.29 eV. Thus it is confirmedthat the band gap in the vicinities of the facet is broadened mainly bythe elimination of InAs in the active layer and the facet is transparentto the emission wavelength.

EXAMPLE 7

A wafer of the same structure as that of Example 6 was employed but thelaser facets were treated in the following manner.

Next, this wafer was cleaved in the atmosphere into laser bars of 700 μmin cavity length and these laser bars were put into a vacuum chamberprovided with an ion-gun to generate Ar plasma. Then the front facet wasfirst irradiated from the side of the facet with heat beam and light byusing a halogen lamp (4.5 KW) for 3 minutes. In this step, the laser barfacet temperature was about 550° C. Further, amorphous Si was deposited(3 nm) on the facet and an AlO_(x) film of 165 nm in thickness wasformed thereafter so that the reflectance of the front facet of theanti-reflective coating layer (15) at emission wavelength of 980 nmbecame 2.5%. In the step of the AlO_(x) film formation, Ar plasma havingaverage energy of 150 eV and current density of 200 μA/cm² and source ofAlO_(x) were supplied to the facet at the same time (IAD method).

Subsequently, the laser bars were once taken out from the vacuum chamberto treat the rear facet. Then the rear facet was treated by irradiatingfrom the side of the facet with heat beam and light by using a halogenlamp (4.5 KW) for 3 minutes. Further, amorphous Si was deposited (3 nm)on the facet and six continuous layers consisting of SiO_(x) film (200nm)/TiO_(x) (120)/SiO_(x) (200 nm)/TiO_(x) (120 nm)/SiO_(x) (200nm)/TiO_(x) (120 nm) were formed to give a rear facet high-reflectivecoating layer (16) having reflectance of 88%.

From these laser bars, five devices were put onto a heat sink andpackaged in a nitrogen atmosphere. These devices showed a thresholdcurrent of 23 mA at 25° C. and kink was observed at 350 mA, 250 mW asthe average initial characteristics. The emission wavelength of thesedevices was 980 nm. When subjected to an accelerated life test of APCmode(150 mW, 70° C.), these samples showed no sudden failure within1,000 hours, as shown in FIG. 19, indicating stable driving.

Similar to Example 6, the band gap in the vicinities of the facet of theactive layer is measured by the electron energy-loss spectroscopy. As aresult, it is found out that the band gap at the InGaAs active layerfacet is 1.45 eV. The energy gap between quantum levels in the InGaAsquantum well active layer at room temperature, determined by measuringthe photoluminescence, is 1.29 eV. Thus it is confirmed that the bandgap in the vicinities of the facet was broadened mainly by theelimination of InAs in the facet of the active layer and the facet istransparent to the emission wavelength.

Neither Ga—O nor As—O is detected when such treated facets are subjectedto XPS measurement with the detection angle of photo-electron of 75°.

Comparative Example 8

The procedure of Example 6 was repeated but the step of the formation ofthe Si passivation layers on the front and rear facets, and the previousstep of selectively exposing the facets to heat beam and light therebymaking the facet transparent were omitted and the coating layers wereformed not by the IAD method but by the conventional electron beamevaporation method. The obtained devices showed a threshold current of21 mA at 25° C. and kink was observed at 350 mA, 250 mW as the averageinitial characteristics, similar to Example 6. When 10 devices weresubjected to a life test of APC mode (150 mW, 70° C.), all of thesamples underwent sudden failure within 150 hours, as shown in FIG. 20.

Both Ga—O and AS—O are detected when the facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAS quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 9

The procedure of Example 6 was repeated but the step of the formation ofthe Si passivation layers on the front and rear facets was omitted andthe coating layers were formed not by the IAD method but by theconventional electron beam evaporation method. The obtained devicesshowed a threshold current of 21 mA at 25° C. and kink was observed at350 mA, 250 mW as the average initial characteristics, similar toExample 6. When 10 devices were subjected to a life test of APC mode(150 mW, 70° C.), four samples underwent sudden failure within 1000hours, as shown in FIG. 21. Moreover, these devices showed a somewhathigher degradation rate than those in Example 6.

As—O is not detected when such treated facets are subjected to XPSmeasurement with the detection angle of photo-electron of 75°.

Comparative Example 10

The procedure of Example 6 was repeated but the step of selectivelyexposing the facets to heat beam and light thereby making the facettransparent, prior to the formation of the Si passivation layers on thefront and rear facets, were omitted, and the coating layers were formednot by IAD method but by conventional electron beam evaporation method.The obtained devices showed a threshold current of 21 mA at 25° C. andkink was observed at 350 mA, 250 mW as the average initialcharacteristics, similar to Example 6. When 10 devices were subjected toa life test of APC mode (150 mW, 70° C.), all of the samples underwentsudden failure within 400 hours, as shown in FIG. 22. Both Ga—O and AS—Oare detected when the facets are subjected to XPS measurement with thedetection angle of photo-electron of 75°.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

Comparative Example 11

The procedure of Example 7 was repeated but the step of the formation ofthe Si passivation layers on the front and rear facets, and the previousstep of selectively exposing the facets to heat beam and light therebymaking the facet transparent were omitted. Further, the coating layerswere formed not by the IAD method but by the conventional electron beamevaporation method. The band gap of the InGaAs quantum well active layermeasured by the electron energy-loss spectroscopy is 1.28 eV which isalmost the same as the value in the bulk region determined by PL. When10 devices thus obtained were subjected to a life test of APC mode (150mW, 70° C.), all of the samples underwent sudden failure within 250hours, as shown in FIG. 23. Both Ga—O and AS—O are detected when thefacets are subjected to XPS measurement with the detection angle ofphoto-electron of 75°.

It is confirmed that the band gap in the vicinities of the facet is thesame as those of InGaAs quantum well or GaAs bulk in the laser diodewhen measured by electron energy-loss spectroscopy.

What is claimed is:
 1. A compound semiconductor light emitting devicecomprising: a substrate; a first conduction type of clad layer formed onthe substrate; an active layer formed on the first conduction type ofclad layer; and a second conduction type of clad layer formed on theactive layer, and two facets of said first conduction type of cladlayer; said active layer, and said second conduction type of clad layer,being opposite to each other so as to form a cavity, the facets andvicinities thereof being high-resistant regions which are transparent tothe emission wavelength, and the surface of the first conduction type ofthe clad layer, active layer and second conduction type of clad layerforming said facets are each coated with a passivation layer.
 2. Acompound semiconductor light emitting device comprising: a substrate; afirst conduction type of clad layer formed on the substrate; a p-typeactive layer formed on the first conduction type of clad layer; and asecond conduction type of clad layer formed on the active layer, and twofacets of said first conduction type of the clad layer; said activelayer; and said second conduction type of clad layer, being opposite toeach other so as to form a cavity, the facets and vicinities thereofbeing high-resistant regions which are transparent to the emissionwavelength, and the surface of the first conduction type the clad layer,active layer and second conduction type of clad layer forming saidfacets are each coated with a Si-containing passivation layer.
 3. Thecompound semiconductor light emitting device as claimed in claim 1,wherein at least one of the surfaces of the first conduction type ofclad layer, active layer and second conduction type of clad layerforming said facets exists in the form free from an oxide.
 4. Thecompound semiconductor light emitting device as claimed in claim 1,wherein the vicinities of said facets have been disordered.
 5. Thecompound semiconductor light emitting device as claimed in claim 1,wherein a coating layer comprising a dielectric material optionallycombined with a semiconductor material is formed of the surface of saidpassivation layer.
 6. The compound semiconductor light emitting deviceas claimed in claim 1, wherein said passivation layer contains Si. 7.The compound semiconductor light emitting device as claimed in claim 1,wherein one of said facets is coated with an anti-reflective coatinglayer containing an AlO, layer while the other is coated with ahigh-reflective coating layer containing AlO, layer and Si layer.
 8. Thecompound semiconductor light emitting device as claimed in claim 1,wherein said active layer comprises a compound semiconductor layercontaining In.
 9. The compound semiconductor light emitting device asclaimed in claim 2, wherein at least one of the constituting elements ofthe surfaces of the first conduction type of clad layer, active layerand second conduction type of clad layer forming said facets exists inthe form free from an oxide.
 10. The compound semiconductor lightemitting device as claimed in claim 2, wherein the vicinities of thefacets of the cavity have been disordered.
 11. The compoundsemiconductor light emitting device as claimed in claim 2, wherein saidactive layer comprises a compound semiconductor layer containing In.